Their kernel partition is too small, the OpenWrt build system
is unable to create an image.

The kernel partition on these devices is too small for the
OpenWrt 19.07 kernel. It is also located after the rootfs, so
we cannot grow it further.

Therefore mark these devices as deprecated now.

This adds documentation for the gluon-mesh-batman-adv package and
elaborates on its build and configuration options, as well as
the implemented multicast architecture.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>

Issue was uncovered by sphinx-build:
docs/user/getting_started.rst:100: WARNING: Definition list ends without a blank line; unexpected unindent.

This new build flag is mandatory for now (it may default to 0 in a future
Gluon version). It may be set to the following values:

* 0 - Do not build any images for deprecated devices.
* upgrade - Only build sysupgrade images for deprecated devices.
* full - Build both sysupgrade and factory images for deprecated devices.

"Other" images are handled like factory images, as they are also used for
the initial installation of Gluon on a device.

Add the `wifi5.outdoor_chanlist` site configuration that
allows specifying an outdoor channel range that can be
switched to for regulatory compliance.

Upon enabling the outdoor option the device will
 - configure the `outdoor_chanlist` on all 5 GHz radios
 - which may enable DFS/TPC, based on the regulatory domain
 - disable ibss/mesh on the 5 GHz radio, as DFS *will*
   break mesh connections
 - allow for htmode reconfiguration on 5 GHz radios

The outdoor option can be toggled from
 - Advanced Settings
   - W-LAN
     - Outdoor Installation

The `preserve_channel` flag overrules the outdoor channel
selection.

A table with hardware supported by Gluon can be found on the OpenWrt Wiki. This commit adds a corresponding FAQ entry.

I just followed this document on a Debian Stretch, and it worked out of
the box. So let's use Debian's current stable release here.